1. Field of the Invention
The present invention relates to a transparent conductive film, a sputtering target and a transparent conductive film-bonded substrate.
2. Description of the Background
A transparent conductive film has both high visible light transmittance and high electrical conductivity and is widely used as a transparent electrode for a display device, such as a liquid crystal display device or a plasma emission device, as a transparent electrode for a solar cell, as a heat reflecting film for an automobile or building glass, as an antistatic film for CRT or as a transparent heater for various antifogging devices, including refrigerating show cases.
As such a transparent conductive film, an ITO (tin-doped indium oxide) film has been mainly employed, since a low resistance film can readily be obtainable, thereby. Particularly, the ITO film is widely used as an electrode for a display device. As other films, a zinc oxide type transparent conductive film and a tin oxide type transparent conductive film of low cost having high chemical resistance are known.
Such conventional transparent conductive film materials are disadvantageous in that with ITO, indium as their main component thereof is expensive and constitutes a bar against cost reduction. With the zinc oxide type transparent conductive film, the chemical resistance thereof against acid or alkali is poor and it is, accordingly, difficult to apply a zinc oxide type transparent conductive film to an industrial product, such as a display device.
The tin oxide type transparent conductive film has excellent chemical resistance, as compared with the ITO film or the zinc oxide type transparent conductive film.
The tin oxide type film is prepared by a spray method or a CVD method as an industrial method for its production, but it is difficult to form the film to have a uniform thickness. Further, during the film formation, chorine, hydrogen chloride, etc. are formed, whereby there used to be an environmental problem due to such a waste gas (or a waste liquid). The tin oxide type transparent conductive film is useful and on the other hand has such various problems.
Further, the tin oxide type transparent conductive film is crystalline, whereby the scratch resistance thereof is poor. The reason for such poor scratch resistance is considered to be due to the face that the film surface has fine irregularities formed during the crystal growth which tend to cause scratching.
In general a sputtering method is suitable for forming a film with a large area whereby a uniform thin film can readily be obtained, and little environmental pollution occurs thereby.
Sputtering methods may generally be classified into a radio frequency (RF) sputtering method using a high frequency power source and a direct current (DC) sputtering method using a direct current power source. The RF sputtering method is excellent in that an electrically insulating material can be used as the target, but the high frequency power source is costly, has a complex structure and is not preferred for forming a film with a large area.
In the DC sputtering method, the target material is limited to a material having good electrical conductivity, but the operation thereof is easy as a direct current power source may be used whereby the required apparatus is simple. As an industrial film-forming method, the DC sputtering method is preferred. JP-A-197315 proposes a method for forming a tin oxide conductive film by a sputtering method, but the description is limited to a RF sputtering method, and there is no description with respect to a DC sputtering method. Further, only a film having a relatively high resistance i.e. the resistivity of the film being at least 8.times.10.sup.-3 .OMEGA.cm, is thereby obtainable.
Further, JP-A-7-335030 proposes a transparent conductive oxide containing one member or a plurality of members selected from the group consisting of In.sub.2 0.sub.3, ZnO, SnO.sub.2 and Ga.sub.2 O.sub.3. However, there is no specific description of a composite oxide containing tin oxide
Further, JP-A-4-272612 proposes an ITO film containing gallium, but indium oxide is the main component, and tin oxide is not the main component.
Thus, a need exists for a tin oxide type-transparent conductive film having both low resistance and high scratch resistance as well as a method for its production.